1. Field of the Invention
This invention relates generally to the field of seismic exploration and more particularly to a seismic data acquisition system and method wherein time invariant encoded signals are injected into the seismic data to ensure correct mapping of the channel assignments of the various detector stations in the system to the true geographic locations of their corresponding detector stations.
2. Description of the Related Art
In seismic exploration, to obtain geophysical information about the earth's sub-surface, shock waves are induced into the earth. The shock waves propagate through the earth's subsurface and are reflected back to the earth's surface by the subterranean interfaces. The reflected seismic waves are detected by a plurality of detectors or receivers implanted on the earth's surface. A geophone or a group of geophones is typically used as a receiver or detector.
In a typical seismic data acquisition system, spaced apart receivers are placed along parallel lines over the terrain to be surveyed. A plurality of remote acquisition units, referred generally herein as "RU", each RU coupled to a certain number of receivers (typically between 1-6), acquire data from their associated receivers. A plurality of data acquisition units, referred generally herein as "DAU", each DAU coupled to a plurality of RU's, receive data from their associated RU's and transmit such received data to a central recording unit, generally referred to herein as a "recorder", which records the data from desired receivers on a storage media during each data acquisition cycle. The recorder is typically placed in a truck or at some other remote location. The recorded data are processed to provide maps of the cross-sections of the earth's subsurface using known techniques.
In recent years, three dimensional ("3D") seismic surveys have become common because they provide more comprehensive geophysical information compared to two dimensional surveys. Three dimensional surveys are typically performed using survey geometries that form a grid, wherein several receiver lines, each receiver line containing a large number spaced apart receivers, are placed in parallel on the terrain to be surveyed. Such layouts or grids may be between 12-30 kilometers long and three to five (3-5) kilometers wide containing more than six thousand (6000) receivers.
To place the receivers and the related equipment on the terrain, a land surveyor stakes out the precise location that each receiver is to occupy. The location is determined by known methods, such as by determining the geographic coordinates of the receiver station by utilizing global positioning systems or other known techniques. The geographic location of each receiver in a seismic spread is typically stored in a computer. Each such receiver provides data over one channel. Thus, in a seismic spread, there are as many data channels as there are receivers and each receiver is associated with one data channel.
Due to the large magnitude and complexity of the modern seismic spreads, it is critical that seismic data acquired from each channel is recorded corresponding to the correct geographic (physical) location of its associated receiver. To accomplish the correct mapping of each channel of seismic data with the correct location of its associated receiver, it is common practice to short a selected number of receivers so that no seismic data is provided by such receivers, i.e., such channels are made "dead." By knowing the actual physical location of such dead channels, the computer correlates the remaining data channels with the known dead channels to ensure that the seismic data (traces) received by the recorder is correctly recorded corresponding to the true geographic location of their associated receivers. This method is flawed in that dead channels frequently exist in seismic spreads due to equipment failure or operator error. Also, valuable seismic data are lost for the receiver stations corresponding to the dead channels, which is very undesirable. Alternatively, in the prior art, the polarity of certain predetermined number of receivers is reversed. This method is undesirable because polarity reversals also naturally occur, mostly due to operator error, and further due to the fact that any such data received by the recorder must by corrected before processing such data. It is thus desirable to have a seismic data acquisition system that provides an independent means for mapping the locations of the data channels to the true geographic locations of their associated receiver stations.
The present invention addresses the above-noted problems and provides a data acquisition system wherein devices placed at certain receiver stations inject time invariant encoded signals into the seismic data, which information is used to ensure that the Recorder records the seismic data corresponding to the true geographic locations of their respective receiver stations.